Washington — Hillary Rodham Clinton has given John McCain a big gift. By winning three of four primaries Tuesday, the New York senator has regained her footing and guaranteed that the Democratic presidential nomination battle will go on for weeks if not months.

Now Senator McCain, the presumed GOP nominee, can work to unify his party, raise money, and sharpen his message against the Democrats. As Senator Clinton and Barack Obama go after each other, McCain and his campaign will be taking careful note of which attacks work best.

"He'll listen to Clinton criticizing Obama and Obama criticizing Clinton, and he'll say that he agrees with both of them," says Darrell West, a political scientist at Brown University in Providence, R.I. The drawn-out nomination fight is "very damaging for the Democrats."

Still, the Democrats head toward the general election with some strong advantages, starting with the weakening economy and the unpopular war in Iraq, which McCain fervently backs. Democrats have also far surpassed the Republicans in fundraising and primary turnout, a sign of their eagerness to put a Democrat in the White House after eight years of George W. Bush. The danger for the Democrats is that their nomination battle could get ugly, leading supporters of the losing candidate to stay home in November.

Democrats argue that their nomination contest ensures that their nominee will be battle-tested for McCain in the fall. Moreover, voters appear to have the appetite for an extended race. According to an ABC News/Washington Post poll released Tuesday, some 67 percent of Democrats and Democratic-leaning independents believed Clinton should stay in the race even if she won just Texas or just Ohio.

"I think voters would be resentful if the party called it off," says Linda Fowler, a government professor at Dartmouth College in Hanover, N.H., noting the high turnout in states unaccustomed to having a role in the primaries. "From the voters' perspective, all this intensive organizing at the grass-roots level is to the good."

Bush's endorsement

As for McCain, the Tuesday primaries handed him a majority of the delegates needed to secure the Republican nomination, prompting former Arkansas Gov. Mick Huckabee to abandon his presidential bid and President Bush to give McCain his official endorsement. Despite the president's low approval rating, his endorsement will help unite the party and win over conservatives skeptical of the independent-minded senator, analysts say.

"John showed incredible courage, strength of character, and perseverance in order to get to this moment, and that's exactly what we need in a president – somebody who can handle the tough decisions, somebody who won't flinch in the face of danger," Mr. Bush said Wednesday, appearing with McCain in the White House Rose Garden.

McCain's return to Washington was a victory lap of sorts after a bruising 16-month Republican presidential primary. He was visiting not only the White House he hopes to occupy but also the Republican National Committee headquarters that he essentially assumes control of now that he's the expected GOP nominee.

Obama's streak ends

With Tuesday's three wins – in Ohio, Texas, and Rhode Island – Clinton reversed an 11-contest losing streak. The seven weeks until the next major primary, in Pennsylvania on April 22, buy her ample time to raise money, sharpen attacks on her rival, and convince the elected officials and party leaders known as superdelegates that she is still a contender.

"She's back," Dr. Fowler says of Clinton. "This gives substance to her claim that she wins in populous states like California, and he wins in the little insignificant 'red' states. It just keeps things very confused."

The day offered Clinton a bright spot ahead of another challenging week. Wyoming's Democrats, who caucus on Saturday, vote in the club of red states Obama has dominated. In Mississippi, which votes March 11, more than half the Democratic primary voters are African-American.

Exit polls Tuesday show that nearly 6 in 10 late-deciding voters broke for Clinton, a sign that her increasingly aggressive tactics may be paying off.

Clinton's harder bite

Over the past week, she has pressed Obama to answer more questions about his ties to Chicago real-estate developer Tony Rezko, a former fundraiser who went on trial this week on federal corruption charges.

She highlighted reports that an Obama adviser allegedly assured Canadian diplomats that Obama's opposition to NAFTA – the North American Free Trade Agreement that is reviled in Ohio and some other struggling industrial states – was more political tactic than policy position.

And she launched a TV ad Feb. 29 featuring sleeping children and asking which candidate voters would rather have answer an emergency phone call at 3 a.m. (Obama is not implicated in the Rezko case, has disputed accounts of his adviser's NAFTA remarks, and has accused Clinton of trying to scare voters with the so-called "red phone" ad.)

"The Clinton campaign believes that not only have they checked Obama's momentum, they've found some themes they can prosecute against him," says Cal Jillson, a political scientist at Southern Methodist University in Dallas. "It will be up to Obama to develop some very credible responses."

Not only did the pillars of her base – white women, older voters, and Hispanics ­– stay loyal Tuesday, but she appeared to put the brakes on Obama's advances among white men. According to exit polls in Ohio, Rhode Island, and Texas, the candidates split independents, a group that typically has favored Obama.

Her margin of victory in Ohio, as in Rhode Island, was convincing: She took 54 percent of the Ohio vote to Obama's 44 percent, and in Rhode Island won 58 percent to 40 percent. The Texas race was nip and tuck, with Clinton at 51 percent and Obama at 47. Obama won big in Vermont, 60 percent to 38 percent.

Another new wrinkle for the Democrats is the possibility of "do-over" primaries in Florida and Michigan. Both states held their primaries earlier than the national party's rules allowed, and they were stripped of their delegates.

While Clinton "won" both primaries, the Democrats cannot seat her delegates without igniting all-out war in the party. But Democrats are hard-put to alienate their voters in two key battleground states by having no delegates seated at the Denver convention.

There are obvious reasons for the Clinton team, especially, to find a solution to this dilemma. The demographics of Florida, in particular, play to Clinton's benefit, having many older and women voters, plus transplants from New York, Clinton's home turf.

Michigan would be a tougher call. It is the most economically stressed state in the US, which plays to the same working-class voter base that handed Clinton a victory in Ohio, but it also has a large black population, which helps Obama.

Despite Clinton's victories on March 4, she still trails in the delegate count and would have to win by massive margins in the remaining contests to close the gap. Holding new primaries in Michigan and Florida sometime this spring may be the only way Clinton can make up her deficit in the tally of "pledged" delegates, those earned in primaries and caucuses.